Energy Conversion: How Life Makes a Living

Abstract

While general ideas about metabolism and respiration were part of scientific thinking for centuries, until relatively recently, no hypothesis could provide a mechanistic explanation for the central process of energy conversion. Respiration and the oxidation of carbon compounds were clearly linked to the synthesis of ATP, but a direct connection remained elusive. In the early 1960s, three competing hypotheses attempted to explain this mechanism: (1) chemical coupling, (2) conformational coupling, and (3) chemiosmosis. In broad outline, hypothesis (3) was ultimately supported, although not without incorporating some aspects of (2). Meanwhile, with the discovery of quantum electron transfer and later supercomplex formation, chemiosmotic electron transfer was shown to proceed extraordinarily rapidly, much faster than soluble chemical reactions. Under favorable conditions, chemiosmosis thus rapidly forms products, and if these products accumulate to the extent that they inhibit electron flow, electrons may divert to molecular oxygen, forming reactive oxygen species (i.e., partially reduced forms of oxygen). While such reactive oxygen species have numerous signaling functions, in large quantities, they pose risks of cellular and organismal damage. By separating hydrogen atoms into protons and electrons, the chemiosmotic process itself is prone to forming dangerous by-products. This has important implications regarding the “management” of chemiosmosis and its products.

In broadest terms, chemiosmosis is the movement of protons over a membrane (hence the resemblance in name to osmosis, the movement of water over a membrane). In respiration, what happens is this. Electrons are stripped from food and passed along a chain of carriers to oxygen. The energy released at several points is used to pump protons across a membrane. The outcome is a proton gradient over the membrane. The membrane acts a bit like a hydroelectric dam. Just as water flowing down from a hilltop reservoir drives a turbine to generate electricity, so in cells the flow of protons through protein turbines in the membrane drives the synthesis of ATP. This mechanism was totally unexpected: instead of having a nice straightforward reaction between two molecules, a strange gradient of protons is interpolated in the middle.

Nick Lane [1]